It has been known that a magnetic structure called the “domain wall” appears within a ferromagnetic thin wire having a cross section measuring a few nanometers to several tens of nanometers in diameter. It has also been known that applying a magnetic field or electric current to a ferromagnetic thin wire having domain walls causes the domain walls to move through the thin wire. Particularly, as compared to the case of controlling the position of the domain walls by applying a magnetic field, the technique of controlling the position of the domain walls by a current supply, as described in Non-Patent Document 1, is advantageous in that the structure of the element can be much simplified.
When a domain wall is moved by supplying a current to a ferromagnetic thin wire, the moving velocity of the domain wall increases in proportion to an increase in the current density. However, the moving velocity suddenly falls if the current density exceeds a certain level. FIG. 9 is a graph showing the relationship between the current density and the moving velocity of a domain wall for different cross-sectional shapes. The graph clearly shows how the moving velocity abruptly decreases. As can be understood from FIG. 9, the threshold at which the abrupt decrease in the moving velocity occurs becomes lower as the cross-sectional shape of the ferromagnetic thin wire approaches from a rectangle to a square; the threshold disappears when the cross section is a complete square.
It has also been known that, after the abrupt decrease in the moving velocity of the domain wall, the magnetic moment begins to rotate at the center of the domain wall as the domain wall moves in the axial direction of the thin wire. (This rotation constantly occurs if the cross-sectional shape is square). FIG. 10 shows how the magnetic moment rotates in a domain wall moving through a ferromagnetic thin wire. Each of the four rectangular sections enclosed by the dotted lines in FIG. 10, having an array of small arrows, represents a part of the ferromagnetic thin wire across a domain wall. The horizontal direction in the each section, indicates the axial (longitudinal) direction of the ferromagnetic thin wire, and the array of small arrows indicates the direction of the magnetic moment. As indicated by the array of small arrows, the magnetic moments in the left and right halves of the each section face each other across the center, where the boundary is the domain wall. In the upper and lower sections, the directions of the magnetic moments in the domain walls are toward the upper side and the downside, respectively. In the left and right sections, the directions of the magnetic moments of the domain walls are toward the front side and the back side of the sheet, respectively. As indicated by the large arrows, the state of the magnetic moment in the domain wall changes with time. As is evident from FIG. 10, the magnetic moment rotates in a sectional plane and around the longitudinal axis of the ferromagnetic thin wire.    Non-Patent Document 1: Akinobu Yamaguchi, et al., “Supin Toransufaa Kouka Ni Yoru Jiheki No Denryuu Kudou (Current-Driven Domain Wall Motion Due to the Spin-Transfer Effect)”, Nihon Ouyou Jikigakkai Shi (Journal of Applied Magnetics Society of Japan), Vol. 28, No. 3, 2004, pp. 343-346    Non-Patent Document 2: Kiselev, S. I. et al. “Microwave oscillations of a nanomagnet driven by a spin-polarized current” Nature, 425, 380-383 (2003)